[RFC] Experimental DMA-BUF Device Heaps
Daniel Vetter
daniel at ffwll.ch
Tue Sep 8 08:36:40 UTC 2020
On Tue, Sep 08, 2020 at 08:43:59AM +0300, Laurent Pinchart wrote:
> Hi Daniel,
>
> On Tue, Sep 01, 2020 at 09:32:22AM +0200, Daniel Vetter wrote:
> > On Sun, Aug 16, 2020 at 02:22:46PM -0300, Ezequiel Garcia wrote:
> > > This heap is basically a wrapper around DMA-API dma_alloc_attrs,
> > > which will allocate memory suitable for the given device.
> > >
> > > The implementation is mostly a port of the Contiguous Videobuf2
> > > memory allocator (see videobuf2/videobuf2-dma-contig.c)
> > > over to the DMA-BUF Heap interface.
> > >
> > > The intention of this allocator is to provide applications
> > > with a more system-agnostic API: the only thing the application
> > > needs to know is which device to get the buffer for.
> > >
> > > Whether the buffer is backed by CMA, IOMMU or a DMA Pool
> > > is unknown to the application.
> > >
> > > I'm not really expecting this patch to be correct or even
> > > a good idea, but just submitting it to start a discussion on DMA-BUF
> > > heap discovery and negotiation.
> > >
> > > Given Plumbers is just a couple weeks from now, I've submitted
> > > a BoF proposal to discuss this, as perhaps it would make
> > > sense to discuss this live?
> > >
> > > Not-signed-off-by: Ezequiel Garcia <ezequiel at collabora.com>
> >
> > I think on the uapi/constraint solving side there's been already tons of
> > discussions while I enjoyed vacations, so different concern from me
> > entirely on the implementation side:
> >
> > In the past the only thing we had in upstream was subsystem/driver
> > specific allocators, and dma-buf for sharing. With dma-buf heaps we kinda
> > get a central allocator, which duplicates large chunks of of all these
> > allocators. And since it's a central allocator by design, the reason for
> > having per-subsystem allocators is kinda gone.
> >
> > I think there's two approaches here:
> > - we convert e.g. drm allocator helpers to internally use the right heap
> > implementation.
>
> We however don't always have a need for a dmabuf, as not all buffers are
> meant to be shared (and we often can't tell beforehand, at allocation
> time, if a given buffer will be shared or not). While the overhead of
> allocating a dmabuf should be file, assigning a file descriptor to each
> buffer would be prohibitely expensive.
>
> We would need to decouple the dma heaps from file descriptors. I think
> that's doable, but it changes the philosophy of dma heaps and needs
> careful consideration. In particular, one may wonder what that would
> bring us, compared to the DMA mapping API for instance.
Oh I don't mean that we use the dma-buf heaps interfaces, but the backend.
Underneath it's all a struct dma_buf, that's where I figured we should
intercept the code sharing. Definitely not file descriptor handles.
> > That would also give us some fairly direct way to expose
> > these constraints in sysfs so a userspace allocator knows which dma-buf
> > heap to pick for shared stuff.
>
> sysfs won't work I'm afraid, as constraints may depend on the format for
> instace. We need subsystem-specific APIs to expose constraints.
Yeah sysfs would be the default, and maybe a list of useable heaps that
the subsystem api then tells you which to use when.
-Daniel
>
> > - we require that any heap is just a different uapi for an existing driver
> > allocator, e.g. by having a dma-buf wrapper for all gem drivers.
> >
> > Otherwise I think what we end up with is a pile of dma-buf heaps for
> > android's blob gpu driver world, and not used anywhere else. Not something
> > even remotely interesting for upstream :-)
> >
> > tldr; I'd like to see how dma-buf heaps closely integrate with all the
> > existing buffer management code we have. Both kernel (and throuhg some
> > allocator library effort) in userspace.
> >
> > > ---
> > > drivers/dma-buf/heaps/Kconfig | 9 +
> > > drivers/dma-buf/heaps/Makefile | 1 +
> > > drivers/dma-buf/heaps/device_heap.c | 268 ++++++++++++++++++++++++++++
> > > include/linux/device.h | 5 +
> > > include/linux/dma-heap.h | 6 +
> > > 5 files changed, 289 insertions(+)
> > > create mode 100644 drivers/dma-buf/heaps/device_heap.c
> > >
> > > diff --git a/drivers/dma-buf/heaps/Kconfig b/drivers/dma-buf/heaps/Kconfig
> > > index a5eef06c4226..2bb3604184bd 100644
> > > --- a/drivers/dma-buf/heaps/Kconfig
> > > +++ b/drivers/dma-buf/heaps/Kconfig
> > > @@ -12,3 +12,12 @@ config DMABUF_HEAPS_CMA
> > > Choose this option to enable dma-buf CMA heap. This heap is backed
> > > by the Contiguous Memory Allocator (CMA). If your system has these
> > > regions, you should say Y here.
> > > +
> > > +config DMABUF_HEAPS_DEVICES
> > > + bool "DMA-BUF Device DMA Heap (Experimental)"
> > > + depends on DMABUF_HEAPS
> > > + help
> > > + Choose this option to enable dma-buf per-device heap. This heap is backed
> > > + by the DMA-API and it's an Experimental feature, meant mostly for testing
> > > + and experimentation.
> > > + Just say N here.
> > > diff --git a/drivers/dma-buf/heaps/Makefile b/drivers/dma-buf/heaps/Makefile
> > > index 6e54cdec3da0..c691d85b3044 100644
> > > --- a/drivers/dma-buf/heaps/Makefile
> > > +++ b/drivers/dma-buf/heaps/Makefile
> > > @@ -2,3 +2,4 @@
> > > obj-y += heap-helpers.o
> > > obj-$(CONFIG_DMABUF_HEAPS_SYSTEM) += system_heap.o
> > > obj-$(CONFIG_DMABUF_HEAPS_CMA) += cma_heap.o
> > > +obj-$(CONFIG_DMABUF_HEAPS_DEVICES) += device_heap.o
> > > diff --git a/drivers/dma-buf/heaps/device_heap.c b/drivers/dma-buf/heaps/device_heap.c
> > > new file mode 100644
> > > index 000000000000..1803dc622dd8
> > > --- /dev/null
> > > +++ b/drivers/dma-buf/heaps/device_heap.c
> > > @@ -0,0 +1,268 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/*
> > > + * DMABUF Device DMA heap exporter
> > > + *
> > > + * Copyright (C) 2020, Collabora Ltd.
> > > + *
> > > + * Based on:
> > > + * videobuf2-dma-contig.c - DMA contig memory allocator for videobuf2
> > > + * Copyright (C) 2010 Samsung Electronics
> > > + */
> > > +
> > > +#include <linux/device.h>
> > > +#include <linux/dma-buf.h>
> > > +#include <linux/dma-heap.h>
> > > +#include <linux/dma-mapping.h>
> > > +#include <linux/scatterlist.h>
> > > +#include <linux/slab.h>
> > > +#include <linux/module.h>
> > > +
> > > +struct dev_dmabuf_attachment {
> > > + struct sg_table sgt;
> > > + enum dma_data_direction dma_dir;
> > > +};
> > > +
> > > +struct dev_dmabuf {
> > > + struct dma_heap *heap;
> > > + struct dma_buf *dmabuf;
> > > + struct device *dev;
> > > + size_t size;
> > > + void *vaddr;
> > > + dma_addr_t dma_addr;
> > > + unsigned long attrs;
> > > +
> > > + struct sg_table sgt;
> > > +};
> > > +
> > > +static struct sg_table *dev_dmabuf_ops_map(struct dma_buf_attachment *db_attach,
> > > + enum dma_data_direction dma_dir)
> > > +{
> > > + struct dev_dmabuf_attachment *attach = db_attach->priv;
> > > + /* stealing dmabuf mutex to serialize map/unmap operations */
> > > + struct mutex *lock = &db_attach->dmabuf->lock;
> > > + struct sg_table *sgt;
> > > +
> > > + mutex_lock(lock);
> > > +
> > > + sgt = &attach->sgt;
> > > + /* return previously mapped sg table */
> > > + if (attach->dma_dir == dma_dir) {
> > > + mutex_unlock(lock);
> > > + return sgt;
> > > + }
> > > +
> > > + /* release any previous cache */
> > > + if (attach->dma_dir != DMA_NONE) {
> > > + dma_unmap_sg_attrs(db_attach->dev, sgt->sgl, sgt->orig_nents,
> > > + attach->dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
> > > + attach->dma_dir = DMA_NONE;
> > > + }
> > > +
> > > + /*
> > > + * mapping to the client with new direction, no cache sync
> > > + * required see comment in .dmabuf_ops_detach()
> > > + */
> > > + sgt->nents = dma_map_sg_attrs(db_attach->dev, sgt->sgl, sgt->orig_nents,
> > > + dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
> > > + if (!sgt->nents) {
> > > + dev_err(db_attach->dev, "failed to map scatterlist\n");
> > > + mutex_unlock(lock);
> > > + return ERR_PTR(-EIO);
> > > + }
> > > +
> > > + attach->dma_dir = dma_dir;
> > > +
> > > + mutex_unlock(lock);
> > > +
> > > + return sgt;
> > > +}
> > > +
> > > +static void dev_dmabuf_ops_unmap(struct dma_buf_attachment *db_attach,
> > > + struct sg_table *sgt,
> > > + enum dma_data_direction dma_dir)
> > > +{
> > > + /* nothing to be done here */
> > > +}
> > > +
> > > +static int dev_dmabuf_ops_attach(struct dma_buf *dmabuf,
> > > + struct dma_buf_attachment *dbuf_attach)
> > > +{
> > > + struct dev_dmabuf_attachment *attach;
> > > + unsigned int i;
> > > + struct scatterlist *rd, *wr;
> > > + struct sg_table *sgt;
> > > + struct dev_dmabuf *buf = dmabuf->priv;
> > > + int ret;
> > > +
> > > + attach = kzalloc(sizeof(*attach), GFP_KERNEL);
> > > + if (!attach)
> > > + return -ENOMEM;
> > > + sgt = &attach->sgt;
> > > +
> > > + /*
> > > + * Copy the buf->sgt scatter list to the attachment, as we can't
> > > + * map the same scatter list to multiple attachments at the same time.
> > > + */
> > > + ret = sg_alloc_table(sgt, buf->sgt.orig_nents, GFP_KERNEL);
> > > + if (ret) {
> > > + kfree(attach);
> > > + return -ENOMEM;
> > > + }
> > > +
> > > + rd = buf->sgt.sgl;
> > > + wr = sgt->sgl;
> > > + for (i = 0; i < sgt->orig_nents; ++i) {
> > > + sg_set_page(wr, sg_page(rd), rd->length, rd->offset);
> > > + rd = sg_next(rd);
> > > + wr = sg_next(wr);
> > > + }
> > > +
> > > + attach->dma_dir = DMA_NONE;
> > > + dbuf_attach->priv = attach;
> > > +
> > > + return 0;
> > > +}
> > > +
> > > +static void dev_dmabuf_ops_detach(struct dma_buf *dmabuf,
> > > + struct dma_buf_attachment *db_attach)
> > > +{
> > > + struct dev_dmabuf_attachment *attach = db_attach->priv;
> > > + struct sg_table *sgt;
> > > +
> > > + if (!attach)
> > > + return;
> > > + sgt = &attach->sgt;
> > > +
> > > + /* release the scatterlist cache */
> > > + if (attach->dma_dir != DMA_NONE)
> > > + /*
> > > + * Cache sync can be skipped here, as the memory is
> > > + * allocated from device coherent memory, which means the
> > > + * memory locations do not require any explicit cache
> > > + * maintenance prior or after being used by the device.
> > > + *
> > > + * XXX: This needs a revisit.
> > > + */
> > > + dma_unmap_sg_attrs(db_attach->dev, sgt->sgl, sgt->orig_nents,
> > > + attach->dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
> > > + sg_free_table(sgt);
> > > + kfree(attach);
> > > + db_attach->priv = NULL;
> > > +}
> > > +
> > > +
> > > +static void *dev_dmabuf_ops_vmap(struct dma_buf *dmabuf)
> > > +{
> > > + struct dev_dmabuf *buf = dmabuf->priv;
> > > +
> > > + return buf->vaddr;
> > > +}
> > > +
> > > +static void dev_dmabuf_ops_release(struct dma_buf *dmabuf)
> > > +{
> > > + struct dev_dmabuf *buf = dmabuf->priv;
> > > +
> > > + sg_free_table(&buf->sgt);
> > > + dma_free_attrs(buf->dev, buf->size, buf->vaddr,
> > > + buf->dma_addr, buf->attrs);
> > > + put_device(buf->dev);
> > > + kfree(buf);
> > > +}
> > > +
> > > +static int dev_dmabuf_ops_mmap(struct dma_buf *dmabuf,
> > > + struct vm_area_struct *vma)
> > > +{
> > > + struct dev_dmabuf *buf = dmabuf->priv;
> > > + int ret;
> > > +
> > > + ret = dma_mmap_attrs(buf->dev, vma, buf->vaddr,
> > > + buf->dma_addr, buf->size,
> > > + buf->attrs);
> > > + if (ret) {
> > > + dev_err(buf->dev, "remapping memory failed, error: %d\n", ret);
> > > + return ret;
> > > + }
> > > + vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
> > > +
> > > + return 0;
> > > +}
> > > +
> > > +static const struct dma_buf_ops dev_dmabuf_ops = {
> > > + .attach = dev_dmabuf_ops_attach,
> > > + .detach = dev_dmabuf_ops_detach,
> > > + .map_dma_buf = dev_dmabuf_ops_map,
> > > + .unmap_dma_buf = dev_dmabuf_ops_unmap,
> > > + .vmap = dev_dmabuf_ops_vmap,
> > > + .mmap = dev_dmabuf_ops_mmap,
> > > + .release = dev_dmabuf_ops_release,
> > > +};
> > > +
> > > +static int dev_heap_allocate(struct dma_heap *heap,
> > > + unsigned long size,
> > > + unsigned long fd_flags,
> > > + unsigned long heap_flags)
> > > +{
> > > + struct device *dev = dma_heap_get_drvdata(heap);
> > > + struct dev_dmabuf *buf;
> > > + struct dma_buf_export_info exp_info = {};
> > > + unsigned long attrs = 0;
> > > + int ret = -ENOMEM;
> > > +
> > > + buf = kzalloc(sizeof(*buf), GFP_KERNEL);
> > > + if (!buf)
> > > + return -ENOMEM;
> > > +
> > > + buf->vaddr = dma_alloc_attrs(dev, size, &buf->dma_addr,
> > > + GFP_KERNEL, attrs);
> > > + /* Prevent the device from being released while the buffer is used */
> > > + buf->dev = get_device(dev);
> > > + buf->heap = heap;
> > > + buf->size = size;
> > > + buf->attrs = attrs;
> > > +
> > > + /* XXX: This call is documented as unsafe. See dma_get_sgtable_attrs(). */
> > > + ret = dma_get_sgtable_attrs(buf->dev, &buf->sgt,
> > > + buf->vaddr, buf->dma_addr,
> > > + buf->size, buf->attrs);
> > > + if (ret < 0) {
> > > + dev_err(buf->dev, "failed to get scatterlist from DMA API\n");
> > > + return ret;
> > > + }
> > > +
> > > + exp_info.exp_name = dev_name(dev);
> > > + exp_info.owner = THIS_MODULE;
> > > + exp_info.ops = &dev_dmabuf_ops;
> > > + exp_info.size = size;
> > > + exp_info.flags = fd_flags;
> > > + exp_info.priv = buf;
> > > +
> > > + buf->dmabuf = dma_buf_export(&exp_info);
> > > + if (IS_ERR(buf->dmabuf)) {
> > > + dev_err(buf->dev, "failed to export dmabuf\n");
> > > + return PTR_ERR(buf->dmabuf);
> > > + }
> > > +
> > > + ret = dma_buf_fd(buf->dmabuf, fd_flags);
> > > + if (ret < 0) {
> > > + dev_err(buf->dev, "failed to get dmabuf fd: %d\n", ret);
> > > + return ret;
> > > + }
> > > +
> > > + return ret;
> > > +}
> > > +
> > > +static const struct dma_heap_ops dev_heap_ops = {
> > > + .allocate = dev_heap_allocate,
> > > +};
> > > +
> > > +void dev_dma_heap_add(struct device *dev)
> > > +{
> > > + struct dma_heap_export_info exp_info;
> > > +
> > > + exp_info.name = dev_name(dev);
> > > + exp_info.ops = &dev_heap_ops;
> > > + exp_info.priv = dev;
> > > +
> > > + dev->heap = dma_heap_add(&exp_info);
> > > +}
> > > +EXPORT_SYMBOL(dev_dma_heap_add);
> > > diff --git a/include/linux/device.h b/include/linux/device.h
> > > index ca18da4768e3..1fae95d55ea1 100644
> > > --- a/include/linux/device.h
> > > +++ b/include/linux/device.h
> > > @@ -45,6 +45,7 @@ struct iommu_ops;
> > > struct iommu_group;
> > > struct dev_pin_info;
> > > struct dev_iommu;
> > > +struct dma_heap;
> > >
> > > /**
> > > * struct subsys_interface - interfaces to device functions
> > > @@ -597,6 +598,10 @@ struct device {
> > > struct iommu_group *iommu_group;
> > > struct dev_iommu *iommu;
> > >
> > > +#ifdef CONFIG_DMABUF_HEAPS_DEVICES
> > > + struct dma_heap *heap;
> > > +#endif
> > > +
> > > bool offline_disabled:1;
> > > bool offline:1;
> > > bool of_node_reused:1;
> > > diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h
> > > index 454e354d1ffb..dcf7cca2f487 100644
> > > --- a/include/linux/dma-heap.h
> > > +++ b/include/linux/dma-heap.h
> > > @@ -56,4 +56,10 @@ void *dma_heap_get_drvdata(struct dma_heap *heap);
> > > */
> > > struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info);
> > >
> > > +#ifdef CONFIG_DMABUF_HEAPS_DEVICES
> > > +void dev_dma_heap_add(struct device *dev);
> > > +#else
> > > +static inline void dev_dma_heap_add(struct device *dev) {}
> > > +#endif
> > > +
> > > #endif /* _DMA_HEAPS_H */
>
> --
> Regards,
>
> Laurent Pinchart
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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